Thermal conductive sheet

ABSTRACT

A thermal conductive sheet contains a plate-like boron nitride particle. The proportion of the boron nitride particle content is 35 vol % or more, and the thermal conductivity in a direction perpendicular to the thickness direction of the thermal conductive sheet is 4 W/m·K or more.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent Applications No. 2010-018256 filed on Jan. 29, 2010; No. 2010-090908 filed on Apr. 9, 2010; No. 2010-161845 filed on Jul. 16, 2010; No. 2010-161847 filed on Jul. 16, 2010; No. 2010-161849 filed on Jul. 16, 2010; No. 2010-161850 filed on Jul. 16, 2010; and No. 2010-161853 filed on Jul. 16, 2010, the contents of which are hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal conductive sheet, to be specific, to a thermal conductive sheet for use in power electronics technology.

2. Description of Related Art

In recent years, power electronics technology which uses semiconductor elements to convert and control electric power is applied in hybrid devices, high-brightness LED devices, and electromagnetic induction heating devices. In power electronics technology, a high current is converted to, for example, heat, and therefore materials that are disposed near the semiconductor element are required to have excellent heat dissipation characteristics (excellent heat conductivity).

For example, Japanese Unexamined Patent Publication No. 2008-280496 has proposed a thermal conductive sheet containing a plate-like boron nitride powder and an acrylic acid ester copolymer resin.

In the thermal conductive sheet of Japanese Unexamined Patent Publication No. 2008-280496, the boron nitride powder is oriented so as to orient its major axis direction (direction perpendicular to the plate thickness of the boron nitride powder) in the thickness direction of the sheet, and thermal conductivity in the thickness direction of the thermal conductive sheet is improved in this way.

SUMMARY OF THE INVENTION

However, there are cases where the thermal conductive sheet is required to have a high thermal conductivity in a direction (plane direction) perpendicular to the thickness direction depending on its use and purpose. In such a case, the thermal conductive sheet of Japanese Unexamined Patent Publication No. 2008-280496 is disadvantageous in that the major axis direction of the boron nitride powder is perpendicular to (crossing) the plane direction, and therefore the thermal conductivity in the plane direction is insufficient.

In addition, thermal conductive sheets are also required to have excellent flexibility, in view of handleability.

An object of the present invention is to provide a thermal conductive sheet having excellent flexibility and thermal conductivity in a plane direction.

A thermal conductive sheet of the present invention contains a plate-like boron nitride particle, wherein the proportion of the boron nitride particle content is 35 vol % or more, and the thermal conductivity in a direction perpendicular to the thickness direction of the thermal conductive sheet is 4 W/m·K or more.

It is preferable that, in the thermal conductive sheet of the present invention, the boron nitride particle has an average particle size as measured by a light scattering method of 20 μm or more.

It is preferable that, in the thermal conductive sheet of the present invention, no fracture is observed in the thermal conductive sheet when the thermal conductive sheet is evaluated in a bend test in conformity with the cylindrical mandrel method of JIS K 5600-5-1 with the test conditions below:

Test Conditions:

Test Device: Type I

Mandrel: diameter 10 mm

Bending Angle: 90 degrees or more

Thickness of the Thermal Conductive Sheet: 0.3 mm

It is preferable that the thermal conductive sheet of the present invention further contains a resin component, wherein the resin component has a kinetic viscosity as measured by the kinetic viscosity test (temperature: 25° C.±0.5° C., solvent: butyl carbitol, solid content concentration: 40 mass %) in conformity with JIS K 7233 (bubble viscometer method) of 0.22×10⁻⁴ to 2.00×10⁻⁴ m²/s.

The thermal conductive sheet of the present invention is excellent in flexibility and thermal conductivity in a plane direction perpendicular to the thickness direction.

Thus, the thermal conductive sheet of the present invention can be used for various heat dissipation applications as a thermal conductive sheet that is excellent in handleability, as well as excellent in thermal conductivity in the plane direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of an embodiment of a thermal conductive sheet of the present invention.

FIG. 2 shows process drawings for describing a method for producing the thermal conductive sheet shown in FIG. 1:

(a) illustrating a step of hot pressing a mixture or a laminated sheet,

(b) illustrating a step of dividing the pressed sheet into a plurality of pieces, and

(c) illustrating a step of laminating the divided sheets.

FIG. 3 shows a processed SEM image of a cross section along the thickness direction of the thermal conductive sheet after curing in Example 1.

FIG. 4 shows a processed SEM image of a cross section along the thickness direction of the thermal conductive sheet after curing in Example 3.

FIG. 5 shows a processed SEM image of a cross section along the thickness direction of the thermal conductive sheet after curing in Example 5.

FIG. 6 shows a processed SEM image of a cross section along the thickness direction of the thermal conductive sheet after curing in Comparative Example 1.

FIG. 7 shows a processed SEM image of a cross section along the thickness direction of the thermal conductive sheet after curing in Comparative Example 2.

FIG. 8 is a graph illustrating relationships between the proportion of the boron nitride particle content and a thermal conductivity of the thermal conductive sheet in Examples 1 to 4, and Comparative Examples 1 and 2.

FIG. 9 shows a perspective view of a test device (Type I, before bend test) of a bend test.

FIG. 10 shows a perspective view of a test device (Type I, during bend test) of a bend test.

DETAILED DESCRIPTION OF THE INVENTION

A thermal conductive sheet of the present invention contains boron nitride particles.

To be specific, the thermal conductive sheet contains boron nitride (BN) particles as an essential component, and further contains, for example, a resin component.

The boron nitride particles are formed into a plate-like (or flake-like) shape, and are dispersed so as to be orientated in a predetermined direction (described later) in the thermal conductive sheet.

The boron nitride particles have an average length in the longitudinal direction (maximum length in the direction perpendicular to the plate thickness direction) of, for example, 1 to 100 μm, or preferably 3 to 90 μm. The boron nitride particles have an average length in the longitudinal direction of, 5 μm or more, preferably 10 μm or more, more preferably 20 μm or more, even more preferably 30 μm or more, or most preferably 40 μm or more, and usually has an average length in the longitudinal direction of, for example, 100 μm or less, or preferably 90 μm or less.

The average thickness (the length in the thickness direction of the plate, that is, the length in the short-side direction of the particles) of the boron nitride particles is, for example, 0.01 to 20 μm, or preferably 0.1 to 15 μm.

The aspect ratio (length in the longitudinal direction/thickness) of the boron nitride particles is, for example, 2 to 10000, or preferably 10 to 5000.

The average particle size of the boron nitride particles as measured by a light scattering method is, for example, 5 μm or more, preferably 10 μm or more, more preferably 20 μm or more, particularly preferably 30 μm or more, or most preferably 40 μm or more, and usually is 100 μm or less.

The average particle size as measured by the light scattering method is a volume average particle size measured with a dynamic light scattering type particle size distribution analyzer.

When the average particle size of the boron nitride particles as measured by the light scattering method is below the above-described range, the thermal conductive sheet may become fragile, and handleability may be reduced.

The bulk density (JIS K 5101, apparent density) of the boron nitride particles is, for example, 0.3 to 1.5 g/cm³, or preferably 0.5 to 1.0 g/cm³.

As the boron nitride particles, a commercially available product or processed goods thereof can be used. Examples of commercially available products of the boron nitride particles include the “PT” series (for example, “PT-110”) manufactured by Momentive Performance Materials Inc., and the “SHOBN®UHP” series (for example, “SHOBN®UHP-1”) manufactured by Showa Denko K.K.

The resin component is a component that is capable of dispersing the boron nitride particles, i.e., a dispersion medium (matrix) in which the boron nitride particles are dispersed, including, for example, resin components such as a thermosetting resin component and a thermoplastic resin component.

Examples of the thermosetting resin component include epoxy resin, thermosetting polyimide, phenol resin, urea resin, melamine resin, unsaturated polyester resin, diallyl phthalate resin, silicone resin, and thermosetting urethane resin.

Examples of the thermoplastic resin component include polyolefin (for example, polyethylene, polypropylene, and ethylene-propylene copolymer), acrylic resin (for example, polymethyl methacrylate), polyvinyl acetate, ethylene-vinyl acetate copolymer, polyvinyl chloride, polystyrene, polyacrylonitrile, polyamide (nylon®), polycarbonate, polyacetal, polyethylene terephthalate, polyphenylene oxide, polyphenylene sulfide, polysulfone, polyether sulfone, poly ether ether ketone, polyallyl sulfone, thermoplastic polyimide, thermoplastic urethane resin, polyamino-bismaleimide, polyamide-imide, polyether-imide, bismaleimide-triazine resin, polymethylpentene, fluorine resin, liquid crystal polymer, olefin-vinyl alcohol copolymer, ionomer, polyarylate, acrylonitrile-ethylene-styrene copolymer, acrylonitrile-butadiene-styrene copolymer, and acrylonitrile-styrene copolymer.

These resin components can be used alone or in combination of two or more.

Of the resin components, preferably, epoxy resin is used as the thermosetting resin component, and preferably, polyolefin is used as the thermoplastic resin component.

The epoxy resin is in a state of liquid, semi-solid, or solid under normal temperature.

To be specific, examples of the epoxy resin include aromatic epoxy resins such as bisphenol epoxy resin (for example, bisphenol A epoxy resin, bisphenol F epoxy resin, bisphenol S epoxy resin, hydrogenated bisphenol A epoxy resin, dimer acid-modified bisphenol epoxy resin, and the like), novolak epoxy resin (for example, phenol novolak epoxy resin, cresol novolak epoxy resin, biphenyl epoxy resin, and the like), naphthalene epoxy resin, fluorene epoxy resin (for example, bisaryl fluorene epoxy resin and the like), and triphenylmethane epoxy resin (for example, trishydroxyphenylmethane epoxy resin and the like); nitrogen-containing-cyclic epoxy resins such as triepoxypropyl isocyanurate (triglycidyl isocyanurate) and hydantoin epoxy resin; aliphatic epoxy resin; alicyclic epoxy resin (for example, dicyclo ring-type epoxy resin and the like); glycidylether epoxy resin; and glycidylamine epoxy resin.

These epoxy resins can be used alone or in combination of two or more.

Preferably, a semi-solid epoxy resin is used alone, or more preferably, a semi-solid aromatic epoxy resin is used alone. Examples of those epoxy resins include, in particular, a semi-solid fluorene epoxy resin.

A combination of a liquid epoxy resin and a solid epoxy resin is also preferable, or a combination of a liquid aromatic epoxy resin and a solid aromatic epoxy resin is even more preferable. Examples of such combinations include a combination of a liquid bisphenol epoxy resin and a solid triphenylmethane epoxy resin, and a combination of a liquid bisphenol epoxy resin and a solid bisphenol epoxy resin.

A semi-solid epoxy resin, or a combination of a liquid epoxy resin and a solid epoxy resin improves conformability to irregularities (described later) of the thermal conductive sheet.

The epoxy resin has an epoxy equivalent of, for example, 100 to 1000 g/eqiv., or preferably 180 to 700 g/eqiv., and has a softening temperature (ring and ball test) of, for example, 80° C. or less (to be specific, 20 to 80° C.), or preferably 70° C. or less (to be specific, 35 to 70° C.)

The epoxy resin has a melt viscosity at 80° C. of, for example, 10 to 20000 mPa·s, or preferably 50 to 10000 mPa·s. When two or more epoxy resins are used in combination, the melt viscosity of the mixture of these epoxy resins is set within the above-described range.

Furthermore, when two or more epoxy resins are used in combination, for example, an epoxy resin that is solid under normal temperature, and an epoxy resin that is liquid under normal temperature are used in combination. Furthermore, when two or more epoxy resins are used in combination, for example, a first epoxy resin having a softening temperature of below 45° C., or preferably below 35° C., and a second epoxy resin having a softening temperature of 45° C. or more, or preferably 55° C. or more are used in combination. In this way, the kinetic viscosity (in conformity with JIS K 7233, described later) of the resin component (mixture) can be set to a desired range.

The epoxy resin can also be prepared as an epoxy resin composition containing, for example, an epoxy resin, a curing agent, and a curing accelerator.

The curing agent is a latent curing agent (epoxy resin curing agent) that can cure the epoxy resin by heating, and examples thereof include an imidazole compound, an amine compound, an acid anhydride compound, an amide compound, a hydrazide compound, and an imidazoline compound. In addition to the above-described compounds, a phenol compound, a urea compound, and a polysulfide compound can also be used.

Examples of the imidazole compound include 2-phenyl imidazole, 2-methyl imidazole, 2-ethyl-4-methyl imidazole, and 2-phenyl-4-methyl-5-hydroxymethyl imidazole.

Examples of the amine compound include polyamines such as ethylene diamine, propylene diamine, diethylene triamine, triethylene tetramine, and amine adducts thereof; metha phenylenediamine; diaminodiphenyl methane; and diaminodiphenyl sulfone.

Examples of the acid anhydride compound include phthalic anhydride, maleic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, 4-methyl-hexahydrophthalic anhydride, methyl nadic anhydride, pyromelletic anhydride, dodecenylsuccinic anhydride, dichloro succinic anhydride, benzophenone tetracarboxylic anhydride, and chlorendic anhydride.

Examples of the amide compound include dicyandiamide and polyamide.

An example of the hydrazide compound includes adipic acid dihydrazide.

Examples of the imidazoline compound include methylimidazoline, 2-ethyl-4-methylimidazoline, ethylimidazoline, isopropylimidazoline, 2,4-dimethylimidazoline, phenylimidazoline, undecylimidazoline, heptadecylimidazoline, and 2-phenyl-4-methylimidazoline.

These curing agents can be used alone or in combination of two or more.

A preferable example of the curing agent is an imidazole compound.

Examples of the curing accelerator include tertiary amine compounds such as triethylenediamine and tri-2,4,6-dimethylaminomethylphenol; phosphorus compounds such as triphenylphosphine, tetraphenylphosphoniumtetraphenylborate, and tetra-n-butylphosphonium-o,o-diethylphosphorodithioate; a quaternary ammonium salt compound; an organic metal salt compound; and derivatives thereof. These curing accelerators can be used alone or in combination of two or more.

In the epoxy resin composition, the mixing ratio of the curing agent is, for example, 0.5 to 50 parts by mass, or preferably 1 to 10 parts by mass per 100 parts by mass of the epoxy resin, and the mixing ratio of the curing accelerator is, for example, 0.1 to 10 parts by mass, or preferably 0.2 to 5 parts by mass per 100 parts by mass of the epoxy resin.

The above-described curing agent, and/or the curing accelerator can be prepared and used, as necessary, as a solution, i.e., the curing agent and/or the curing accelerator dissolved in a solvent; and/or as a dispersion liquid, i.e., the curing agent and/or the curing accelerator dispersed in a solvent.

Examples of the solvent include organic solvents including ketones such as acetone and methyl ethyl ketone, ester such as ethyl acetate, and amide such as N,N-dimethylformamide. Examples of the solvent also include aqueous solvents including water, and alcohols such as methanol, ethanol, propanol, and isopropanol. A preferable example is an organic solvent, and a more preferable example is ketone.

Preferable examples of polyolefin are polyethylene and ethylene-propylene copolymer.

Examples of polyethylene include a low density polyethylene and a high density polyethylene.

Examples of ethylene-propylene copolymer include a random copolymer, a block copolymer, or a graft copolymer of ethylene and propylene.

These polyolefins can be used alone or in combination of two or more.

The polyolefins have a weight average molecular weight and/or a number average molecular weight of, for example, 1000 to 10000.

The polyolefin can be used alone, or can be used in combination of two or more.

Of the resin components, preferably, a thermosetting resin component is used, or more preferably, an epoxy resin is used.

The resin component has a kinetic viscosity as measured in conformity with the kinetic viscosity test of JIS K 7233 (bubble viscometer method) (temperature: 25° C.±0.5° C., solvent: butyl carbitol, resin component (solid content) concentration: 40 mass %) of, for example, 0.22×10⁻⁴ to 2.00×10⁻⁴ m²/s, preferably 0.3×10⁻⁴ to 1.9×10⁻⁴ m²/s, or 1.8×10⁻⁴ m²/s. The above-described kinetic viscosity can also be set to, for example, 0.22×10⁻⁴ to 1.00×10⁻⁴ m²/s, preferably 0.3×10⁻⁴ to 0.9×10⁻⁴ m²/s, or 0.8×10⁻⁴ m²/s.

When the kinetic viscosity of the resin component exceeds the above-described range, excellent flexibility and conformability to irregularities (described later) may not be given to the thermal conductive sheet. On the other hand, when the kinetic viscosity of the resin component is below the above-described range, boron nitride particles may not be oriented in a predetermined direction.

In the kinetic viscosity test in conformity with JIS K 7233 (bubble viscometer method), the kinetic viscosity of the resin component is measured by comparing the bubble rising speed of a resin component sample with the bubble rising speed of criterion samples (having a known kinetic viscosity), and determining the kinetic viscosity of the criterion sample having a matching rising speed to be the kinetic viscosity of the resin component.

In the thermal conductive sheet, the proportion of the volume-based boron nitride particle content (solid content, that is, when the resin component includes the thermoplastic resin component, the volume percentage of boron nitride particles relative to a total volume of the thermoplastic resin component and the boron nitride particles) is, 35 vol % or more, preferably 60 vol % or more, or more preferably 75 vol % or more, and usually, for example, 95 vol % or less, or preferably 90 vol % or less.

When the proportion of the volume-based boron nitride particle content is below the above-described range, the boron nitride particles cannot be oriented in a predetermined direction in the thermal conductive sheet. On the other hand, when the proportion of the volume-based boron nitride particle content exceeds the above-described range, the thermal conductive sheet may become fragile, and handleability and conformability to irregularities (described later) may be reduced.

The mass-based mixing ratio of the boron nitride particles relative to 100 parts by mass of the total amount (total solid content) of the components (boron nitride particles and resin component) forming the thermal conductive sheet is, for example, 40 to 95 parts by mass, or preferably 65 to 90 parts by mass, and the mass-based mixing ratio of the resin component relative to 100 parts by mass of the total amount of the components forming the thermal conductive sheet is, for example, 5 to 60 parts by mass, or preferably 10 to 35 parts by mass. The mass-based mixing ratio of the boron nitride particles relative to 100 parts by mass of the resin component is, for example, 60 to 1900 parts by mass, or preferably 185 to 900 parts by mass.

When two epoxy resins (a first epoxy resin and a second epoxy resin) are used in combination, the mass ratio (mass of the first epoxy resin/mass of the second epoxy resin) of the first epoxy resin relative to the second epoxy resin can be set appropriately in accordance with the softening temperature and the like of the epoxy resins (the first epoxy resin and the second epoxy resin). For example, the mass ratio of the first epoxy resin relative to the second epoxy resin is 1/99 to 99/1, or preferably 10/90 to 90/10.

In the resin component, in addition to the above-described components (polymer), for example, a polymer precursor (for example, a low molecular weight polymer including oligomer), and/or a monomer are contained.

FIG. 1 shows a perspective view of an embodiment of a thermal conductive sheet of the present invention, and FIG. 2 shows process drawings for describing a method for producing the thermal conductive sheet shown in FIG. 1.

Next, a method for producing a thermal conductive sheet as an embodiment of the present invention is described with reference to FIG. 1 and FIG. 2.

In this method, first, the above-described components are blended at the above-described mixing ratio and are stirred and mixed, thereby preparing a mixture.

In the stirring and mixing, in order to mix the components efficiently, for example, the solvent may be blended therein with the above-described components, or, for example, the resin component (preferably, the thermoplastic resin component) can be melted by heating.

Examples of the solvent include the above-described organic solvents. When the above-described curing agent and/or the curing accelerator are prepared as a solvent solution and/or a solvent dispersion liquid, the solvent of the solvent solution and/or the solvent dispersion liquid can also serve as a mixing solvent for the stirring and mixing without adding a solvent during the stirring and mixing. Or, in the stirring and mixing, a solvent can be further added as a mixing solvent.

In the case when the stirring and mixing is performed using a solvent, the solvent is removed after the stirring and mixing.

To remove the solvent, for example, the mixture is allowed to stand at room temperature for 1 to 48 hours; heated at 40 to 100° C. for 0.5 to 3 hours; or heated under a reduced pressure atmosphere of, for example, 0.001 to 50 kPa, at 20 to 60° C., for 0.5 to 3 hours.

When the resin component (preferably, a thermoplastic resin component) is to be melted by heating, the heating temperature is, for example, a temperature in the neighborhood of or exceeding the softening temperature of the resin component, to be specific, 40 to 150° C., or preferably 70 to 140° C.

Next, in this method, the obtained mixture is hot-pressed.

To be specific, as shown in FIG. 2( a), as necessary, for example, the mixture is hot-pressed with two releasing films 4 sandwiching the mixture, thereby producing a pressed sheet 1A. Conditions for the hot-pressing are as follows: a temperature of, for example, 50 to 150° C., or preferably 60 to 140° C.; a pressure of, for example, 1 to 100 MPa, or preferably 5 to 50 MPa; and a duration of, for example, 0.1 to 100 minutes, or preferably 1 to 30 minutes.

More preferably, the mixture is hot-pressed under vacuum. The degree of vacuum in the vacuum hot-pressing is, for example, 1 to 100 Pa, or preferably 5 to 50 Pa, and the temperature, the pressure, and the duration are the same as those described above for the hot-pressing.

When the temperature, the pressure, and/or the duration in the hot-pressing is outside the above-described range, there is a case where a porosity P (described later) of the thermal conductive sheet 1 cannot be adjusted to give a desired value.

The pressed sheet 1A obtained by the hot-pressing has a thickness of, for example, 50 to 1000 μm, or preferably 100 to 800 μm.

Next, in this method, as shown in FIG. 20( b), the pressed sheet 1A is divided into a plurality of pieces (for example, four pieces), thereby producing a divided sheet 1B (dividing step). In the division of the pressed sheet 1A, the pressed sheet 1A is cut along the thickness direction so that the pressed sheet 1A is divided into a plurality of pieces when the pressed sheet 1A is projected in the thickness direction. The pressed sheet 1A is cut so that the respective divided sheets 1B have the same shape when the divided sheets 1B are projected in the thickness direction.

Next, in this method, as shown in FIG. 2( c), the respective divided sheets 1B are laminated in the thickness direction, thereby producing a laminated sheet 1C (laminating step).

Thereafter, in this method, as shown in FIG. 2( a), the laminated sheet 1C is hot-pressed (preferably hot-pressed under vacuum) (hot-pressing step). The conditions for the hot-pressing are the same as the conditions for the hot-pressing of the above-described mixture.

The thickness of the hot-pressed laminated sheet 1C is, for example, 1 mm or less, or preferably 0.8 mm or less, and usually is, for example, 0.05 mm or more, or preferably 0.1 mm or more.

Thereafter, the series of the steps of the above-described dividing step (FIG. 2( b)), laminating step (FIG. 2( c)), and hot-pressing step (FIG. 2( a)) are performed repeatedly, so as to allow boron nitride particles 2 to be efficiently oriented in a predetermined direction in the resin component 3 in the thermal conductive sheet 1. The number of the repetition is not particularly limited, and can be set appropriately according to the charging state of the boron nitride particles. The number of the repetition is, for example, 1 to 10 times, or preferably 2 to 7 times.

The thermal conductive sheet 1 can be obtained in this manner.

The thickness of the obtained thermal conductive sheet 1 is, for example, 1 mm or less, or preferably 0.8 mm or less, and usually, for example, 0.05 mm or more, or preferably 0.1 mm or more.

In the thermal conductive sheet 1, the proportion of the volume-based boron nitride particle content (solid content, that is, volume percentage of boron nitride particles relative to the total volume of the resin component and the boron nitride particles) is, as described above, 35 vol % or more (preferably 60 vol % or more, or more preferably 75 vol % or more), and usually 95 vol % or less (preferably 90 vol % or less).

When the proportion of the boron nitride particle content is below the above-described range, the boron nitride particles cannot be blended in a predetermined direction in the thermal conductive sheet.

When the resin component 3 is the thermosetting resin component, an uncured (or semi-cured (in stage B)) thermal conductive sheet 1 is cured by heating after the above-described hot-pressing step (FIG. 2( a)), thereby preparing a cured thermal conductive sheet 1.

To cure the thermal conductive sheet 1 by heat, the above-described hot-press or a dryer is used. Preferably, a dryer is used. The conditions for the curing by heat are as follows: a temperature of, for example, 60 to 250° C., or preferably 80 to 200° C. When the hot-pressing is performed, the pressure is, for example, 100 MPa or less, or preferably 50 MPa or less.

In the thus obtained thermal conductive sheet 1, as shown in FIG. 1 and its partially enlarged schematic view, the longitudinal direction LD of the boron nitride particle 2 is oriented along a plane (surface) direction SD that crosses (is perpendicular to) the thickness direction TD of the thermal conductive sheet 1.

The calculated average of the angle formed between the longitudinal direction LD of the boron nitride particle 2 and the plane direction SD of the thermal conductive sheet 1 (orientation angle a of the boron nitride particles 2 relative to the thermal conductive sheet 1) is, for example, 25 degrees or less, or preferably 20 degrees or less, and usually 0 degree or more.

The orientation angle α of the boron nitride particle 2 relative to the thermal conductive sheet 1 is obtained as follows: the thermal conductive sheet 1 is cut along the thickness direction with a cross section polisher (CP); the cross section thus appeared is photographed with a scanning electron microscope (SEM) at a magnification that enables observation of 200 or more boron nitride particles 2 in the field of view; a tilt angle a between the longitudinal direction LD of the boron nitride particle 2 and the plane direction SD (direction perpendicular to the thickness direction TD) of the thermal conductive sheet 1 is obtained from the obtained SEM photograph; and the average value of the tilt angles a is calculated.

Thus, the thermal conductivity in the plane direction SD of the thermal conductive sheet 1 is 4 W/m or more, preferably 5 W/m·K or more, more preferably 10 W/m·K or more, even more preferably 15 W/m·K or more, or particularly preferably 25 W/m·K or more, and usually 200 W/m·K or less.

The thermal conductivity in the plane direction SD of the thermal conductive sheet 1 is substantially the same before and after the curing by heat when the resin component 3 is the thermosetting resin component.

When the thermal conductivity in the plane direction SD of the thermal conductive sheet 1 is below the above-described range, thermal conductivity in the plane direction SD is insufficient, and therefore there is a case where the thermal conductive sheet 1 cannot be used for heat dissipation that requires thermal conductivity in such a plane direction SD.

The thermal conductivity in the plane direction SD of the thermal conductive sheet 1 is measured by a pulse heating method. In the pulse heating method, the xenonflash analyzer “LFA-447” (manufactured by Erich NETZSCH GmbH & Co. Holding KG) is used.

The thermal conductivity in the thickness direction TD of the thermal conductive sheet 1 is, for example, 0.5 to 15 W/m·K, or preferably 1 to 10 W/m·K.

The thermal conductivity in the thickness direction TD of the thermal conductive sheet 1 is measured by a pulse heating method, a laser flash method, or a TWA method. In the pulse heating method, the above-described device is used, in the laser flash method, “TC-9000” (manufactured by Ulvac, Inc.) is used, and in the TWA method, “ai-Phase mobile” (manufactured by ai-Phase Co., Ltd) is used.

Thus, the ratio of the thermal conductivity in the plane direction SD of the thermal conductive sheet 1 relative to the thermal conductivity in the thickness direction TD of the thermal conductive sheet 1 (thermal conductivity in the plane direction SD/thermal conductivity in the thickness direction TD) is, for example, 1.5 or more, preferably 3 or more, or more preferably 4 or more, and usually 20 or less.

Although not shown in FIG. 1, for example, pores (gaps) are formed in the thermal conductive sheet 1.

The proportion of the pores in the thermal conductive sheet 1, that is, a porosity P, can be adjusted by setting the proportion of the boron nitride particle 2 content (volume-based), and further setting the temperature, the pressure, and/or the duration at the time of hot pressing the mixture of the boron nitride particle 2 and the resin component 3 (FIG. 2( a)). To be specific, the porosity P can be adjusted by setting the temperature, the pressure, and/or the duration of the hot pressing (FIG. 2( a)) within the above-described range.

The porosity P of the thermal conductive sheet 1 is, for example, 30 vol % or less, or preferably 10 vol % or less.

The porosity P is measured by, for example, as follows: the thermal conductive sheet 1 is cut along the thickness direction with a cross section polisher (CP); the cross section thus appeared is observed with a scanning electron microscope (SEM) at a magnification of 200 to obtain an image; the obtained image is binarized based on the pore portion and the non-pore portion; and the area ratio, i.e., the ratio of the pore portion area to the total area of the cross section of the thermal conductive sheet 1 is determined by calculation.

The thermal conductive sheet 1 has a porosity P2 after curing of, relative to a porosity P1 before curing, for example, 100% or less, or preferably 50% or less.

For the measurement of the porosity P (P1), when the resin component 3 is a thermosetting resin component, the thermal conductive sheet 1 before curing by heat is used.

When the porosity P of the thermal conductive sheet 1 is within the above-described range, the conformability to irregularities (described later) of the thermal conductive sheet 1 can be improved.

When the thermal conductive sheet 1 is evaluated in the bend test in conformity with the cylindrical mandrel method of JIS K 5600-5-1 under the test conditions shown below, for example, no fracture is observed.

Test Conditions:

Test Device: Type I

Mandrel: diameter 10 mm

Bending Angle: 90 degrees or more

Thickness of the thermal conductive sheet 1: 0.3 mm

FIGS. 9 and 10 show perspective views of the Type I test device. In the following, the Type I test device is described.

In FIGS. 9 and 10, a Type I test device 10 includes a first flat plate 11; a second flat plate 12 disposed in parallel with the first flat plate 11; and a mandrel (rotation axis) 13 provided for allowing the first flat plate 11 and the second flat plate 12 to rotate relatively.

The first flat plate 11 is formed into a generally rectangular flat plate. A stopper 14 is provided at one end portion (free end portion) of the first flat plate 11. The stopper 14 is formed on the surface of the second flat plate 12 so as to extend along the one end portion of the second flat plate 12.

The second flat plate 12 is formed into a generally rectangular flat plate, and one side thereof is disposed so as to be adjacent to one side (the other end portion (proximal end portion) that is opposite to the one end portion where the stopper 14 is provided) of the first flat plate 11.

The mandrel 13 is formed so as to extend along one side of the first flat plate 11 and one side of the second flat plate 12 that are adjacent to each other.

In the Type I test device 10, as shown in FIG. 9, the surface of the first flat plate 11 is flush with the surface of the second flat plate 12 before the start of the bend test.

To perform the bend test, the thermal conductive sheet 1 is placed on the surface of the first flat plate 11 and the surface of the second flat plate 12. The thermal conductive sheet 1 is placed so that one side of the thermal conductive sheet 1 is in contact with the stopper 14.

Then, as shown in FIG. 10, the first flat plate 11 and the second flat plate 12 are rotated relatively. In particular, the free end portion of the first flat plate 11 and the free end portion of the second flat plate 12 are rotated to a predetermined angle with the mandrel 13 as the center. To be specific, the first flat plate 11 and the second flat plate 12 are rotated so as to bring the surface of the free end portions thereof closer (oppose each other).

In this way, the thermal conductive sheet 1 is bent with the mandrel 13 as the center, conforming to the rotation of the first flat plate 11 and the second flat plate 12.

Preferably, no fracture is observed in the thermal conductive sheet 1 even when the bending angle is set to 180 degrees under the above-described test conditions.

When the fracture is observed in the bend test at the above bending angle in the thermal conductive sheet 1, there is a case where excellent flexibility cannot be given to the thermal conductive sheet 1.

When the resin component 3 is a thermosetting resin component, the thermal conductive sheet 1 before curing by heat is used in the bend test.

Furthermore, for example, when the thermal conductive sheet 1 is evaluated in the 3-point bending test in conformity with JIS K 7171 (2008) under the test conditions shown below, no fracture is observed.

Test Conditions:

Test piece: size 20 mm×15 mm

Distance between supporting points: 5 mm

Testing speed: 20 mm/min (indenter depressing speed)

Bending angle: 120 degrees

Evaluation method: presence or absence of fracture such as cracks at the center of the test piece is observed visually when tested under the above-described test conditions.

In the 3-point bending test, when the resin component 3 is a thermosetting resin component, the thermal conductive sheet 1 before curing by heat is used.

Therefore, the thermal conductive sheet 1 is excellent in conformability to irregularities because no fracture is observed in the above-described 3-point bending test. The conformability to irregularities is, when the thermal conductive sheet 1 is provided on an object with irregularities, a property of the thermal conductive sheet 1 that conforms to be in close contact with the irregularities.

A mark such as, for example, letters and symbols can be adhered to the thermal conductive sheet 1. That is, the thermal conductive sheet 1 is excellent in mark adhesion. The mark adhesion is a property of the thermal conductive sheet 1 that allows reliable adhesion of the above-described mark thereon.

The mark can be adhered (applied, fixed, or firmly fixed) to the thermal conductive sheet 1, to be specific, by printing, engraving, or the like.

Examples of printing include, for example, inkjet printing, relief printing, intaglio printing, and laser printing.

When the mark is to be printed by inkjet printing, relief printing, or intaglio printing, for example, an ink fixing layer for improving mark's fixed state can be provided on the surface (printing side) of the thermal conductive sheet 1.

When the mark is to be printed by laser printing, for example, a toner fixing layer for improving mark's fixed state can be provided on the surface (printing side) of the thermal conductive sheet 1.

Examples of engraving include laser engraving, and punching.

The thermal conductive sheet 1 has a volume resistivity R of, for example, 1×10¹⁰ Ω·cm or more, preferably 1×10¹² Ω·cm or more, and usually 1×10²⁰ Ω·cm or less.

The volume resistivity R of the thermal conductive sheet 1 is measured in conformity with JIS K 6911 (thermosetting plastic general testing method, 2006).

When the thermal conductive sheet 1 has a volume resistivity R below the above-described range, there is a case where short circuits between the electron devices to be described later cannot be prevented.

When the resin component 3 is a thermosetting resin component in the thermal conductive sheet 1, the volume resistivity R is a value of a cured thermal conductive sheet 1.

The thermal conductive sheet 1 has a breakdown voltage measured in conformity with JIS C 2110 (2010) of, for example, 10 kV/mm or more. When the breakdown voltage of the thermal conductive sheet 1 is below 10 kV/mm, there is a case where excellent resistance to electric breakdown (resistance to tracking) cannot be ensured.

The above-described breakdown voltage is measured in conformity with the description of “solid electrical insulating materials—Test methods for electric strength—Part 2: Tests using direct voltage” of JIS C 2110-2 (2010). In detail, the breakdown voltage is determined by measuring a voltage that causes electric breakdown in the thermal conductive sheet 1 in a short period (rapid pressure rise) test using a pressure-rising speed of 1000 V/s.

The thermal conductive sheet 1 has a breakdown voltage of preferably 15 kV/mm or more, and usually has a breakdown voltage of 100 kV/mm or less.

When the resin component 3 is a thermosetting resin component, the breakdown voltage of the thermal conductive sheet 1 is substantially the same before and after curing of the thermal conductive sheet 1.

The thermal conductive sheet 1 has a glass transition temperature of, for example, 125° C. or more, preferably 130° C. or more, more preferably 140° C. or more, more preferably 150° C. or more, more preferably 170° C. or more, more preferably 190° C. or more, or more preferably 210° C. or more, and usually has a glass transition temperature of 300° C. or less.

When the glass transition temperature is the above-described lower limit or more, a thermal conductive sheet with excellent heat resistance can be ensured, and therefore deformation under high temperature can be reduced, and peeling can be suppressed.

That is, when the thermal conductive sheet 1 is bonded to a device of various types and the temperature of the device rises to exceed the glass transition temperature of the thermal conductive sheet 1, the thermal conductive sheet 1 may be peeled off from the device depending on the changes in the linear expansion coefficient. However, in this thermal conductive sheet 1, because the glass transition temperature is equal to or more than the above-described upper limit, even if the device temperature rises, exceeding the glass transition temperature of the thermal conductive sheet 1 can be suppressed, and as a result, deformation of the thermal conductive sheet 1 can be reduced, and the peeling can be suppressed.

The glass transition temperature is obtained by measuring a dynamic viscoelasticity using a frequency of 10 Hz, and determining the peak value of tans (loss tangent).

The thermal conductive sheet 1 has a 5% mass loss temperature of, for example, 250° C. or more, or preferably 300° C. or more, and usually has a 5% mass loss temperature of 450° C. or less.

When the 5% mass loss temperature is the above-described lower limit or more, decomposition can be suppressed even if exposed to high temperature, and heat generated from various devices can be conducted efficiently.

The 5% mass loss temperature can be measured by thermogravimetric analysis (temperature rising speed 10° C./min, under nitrogen atmosphere) in conformity with JIS K 7120.

The thermal conductive sheet 1 does not fall off from, for example, an adherend in the initial adhesion test (1) described below. That is, a temporary fixed state between the thermal conductive sheet 1 and the adherend is kept.

Initial Adhesion Test (1): The thermal conductive sheet 1 is thermocompression bonded on top of an adherend that is placed along a horizontal direction to be temporary fixed thereon, allowed to stand for 10 minutes, and the adherend is turned over to be upside down.

Examples of the adherend include a substrate made of stainless steel (e.g., SUS 304 and the like), or a mounting substrate for notebook PC on which a plurality of electronic components such as IC (integrated circuit) chips, condensers, coils, and resistors are mounted. In the mounting substrate for notebook PC, the electronic components are usually disposed on the top face (one side) with a space provided therebetween in the plane direction (the plane direction of the mounting substrate for notebook PC).

In the pressure bonding, for example, while a sponge roll made of a resin such as silicone resin is pressed against the thermal conductive sheet 1, the sponge roll is rolled on the surface of the thermal conductive sheet 1.

The temperature of the thermocompression bonding is, when the resin component 3 is a thermosetting resin component (for example, epoxy resin), for example, 80° C.

On the other hand, when the resin component 3 is a thermoplastic resin component (for example, polyethylene), the temperature of the thermocompression bonding is a temperature higher by 10 to 30° C. than the softening point or the melting point of the thermoplastic resin component; preferably a temperature higher by 15 to 25° C. than the softening point or the melting point of the thermoplastic resin component; more preferably, a temperature higher by 20° C. than the softening point or the melting point of the thermoplastic resin component; or to be specific, a temperature of 120° C. (that is, the softening point or the melting point of the thermoplastic resin component is 100° C., and the temperature higher by 20° C. than 100° C. is 120° C.).

When the thermal conductive sheet 1 falls off from the adherend in the above-described initial adhesion test (1), that is, when the temporary fixed state between the thermal conductive sheet 1 and the adherend is not kept, there is a case where the thermal conductive sheet 1 cannot be reliably temporary fixed to the adherend.

When the resin component 3 is a thermosetting resin component, the thermal conductive sheet 1 to be tested in the initial adhesion test (1) and the initial adhesion test (2) (described later) is a thermal conductive sheet 1 before curing, and the thermal conductive sheet 1 will be in B-stage based on the thermocompression bonding in the initial adhesion test (1) and the initial adhesion test (2).

When the resin component 3 is a thermoplastic resin component, the thermal conductive sheet 1 to be tested in the initial adhesion test (1) and the initial adhesion test (2) (described later) is a solid thermal conductive sheet 1, and the thermal conductive sheet 1 is softened by the thermocompression bonding in the initial adhesion test (1) and the initial adhesion test (2).

Preferably, the thermal conductive sheet 1 does not fall off from the adherend in both of the above-described initial adhesion test (1) and the initial adhesion test (2) described below. That is, the temporary fixed state between the thermal conductive sheet 1 and the adherend is kept.

Initial Adhesion Test (2): The thermal conductive sheet 1 is thermocompression bonded on top of an adherend that is placed along a horizontal direction to be temporary fixed thereon, and then allowed to stand for 10 minutes, and thereafter, the adherend is disposed along a vertical direction (up-down directions).

The temperature in the thermocompression bonding of the Initial Adhesion Test (2) is the same as the temperature in the Initial Adhesion Test (1).

The thermal conductive sheet 1 is excellent in flexibility and thermal conductivity in the plane direction SD.

Thus, the thermal conductive sheet 1 can be used for various heat dissipation applications, as a thermal conductive sheet that is excellent in handleability and has excellent thermal conductivity in the plane direction SD, to be specific, as a thermal conductive sheet applied in power electronics technology, to be more specific, as a thermal conductive sheet used, for example, as an LED heat dissipation substrate, or as a heat dissipation material for batteries.

The above-described thermal conductive sheet 1 is excellent in thermal conductivity in the plane direction SD, and at the same time, because the thermal conductive sheet 1 has a volume resistivity R in a specific range, it is excellent in electrical insulation.

Therefore, by covering the electron devices with the thermal conductive sheet 1, the electron devices can be protected, the heat from the electron devices can be conducted efficiently, and a short circuit between the electron devices can be prevented.

The electron devices to be covered with the thermal conductive sheet 1 are not particularly limited, and examples thereof include IC (integrated circuit) chips, condensers, coils, resistors, and light-emitting diodes. These electron devices are usually provided on a substrate, and are arranged with a space provided therebetween in the plane direction (plane direction of the substrate).

Furthermore, the above-described thermal conductive sheet 1 is excellent in thermal conductivity in the plane direction SD and has a breakdown voltage in a specific range, and therefore the thermal conductive sheet 1 is also excellently resistant to electric breakdown (resistance to tracking).

Thus, by covering electronic components used for power electronics with the thermal conductive sheet 1, and/or covering the mounting substrate on which these electronic components are mounted, electric breakdown of the thermal conductive sheet 1 can be prevented, and at the same time, the thermal conductive sheet 1 allows the heat from the electronic components, and/or mounting substrate to be dissipated along the plane direction SD.

Examples of the electronic components used for power electronics include IC (integrated circuit) chips (in particular, narrow width portions of electrode terminals in IC chips), thyristors (rectifier), motor parts, inverters, electrical power transmission components, condensers, coils, resistors, and light-emitting diodes.

The above-described electronic components are mounted on the surface (one side) of the mounting substrate, and the electronic components are disposed on the mounting substrate with a space provided therebetween in the plane direction (plane direction of the mounting substrate).

The thermal conductive sheet 1 covering the above-described electronic components and/or mounting substrate can also prevent deterioration caused by, for example, high-frequency noise generated from the electronic component, and/or the mounting substrate.

Furthermore, the above-described thermal conductive sheet 1 is excellent in thermal conductivity in the plane direction SD and also has the glass transition temperature in a specific range, and therefore the thermal conductive sheet 1 is also excellent in heat resistance.

Thus, as a thermal conductive sheet that allows decrease in deformation under high temperature, suppresses peeling, is excellent in handleability, and has excellent thermal conductivity in the plane direction, the thermal conductive sheet can be used for various heat dissipation applications, to be specific, as a thermal conductive sheet applied in power electronics technology, to be more specific, as a thermal conductive sheet used, for example, as an LED heat dissipation substrate, or as a heat dissipation material for batteries.

Furthermore, the above-described thermal conductive sheet 1 is excellent in flexibility and thermal conductivity in the plane direction SD, and also has a 5% mass loss temperature within a specific range, and therefore the thermal conductive sheet 1 is also excellent in heat resistance.

That is, the thermal conductive sheet 1 can be used for various heat dissipation applications, for example, as a thermal conductive sheet that allows suppression of decomposition even if exposed to a high temperature of 200° C. or more, is excellent in handleability, and is excellent in thermal conductivity in the plane direction SD. To be specific, the thermal conductive sheet 1 can be used as a thermal conductive sheet that is applied in power electronics technology generating a high temperature of 200 to 250° C. To be more specific, for example, the thermal conductive sheet 1 can be used as a thermal conductive sheet used for SiC chips, LED heat dissipation substrates, or heat dissipation materials for batteries.

Furthermore, the above-described thermal conductive sheet 1 is excellent in thermal conductivity in the plane direction SD, and at the same time, in the above-described initial adhesion test (1), for example, does not fall off from the adherend, and therefore the thermal conductive sheet 1 also has excellent adhesion (initial adhesion) to the adherend after thermocompression bonding at a predetermined temperature.

Thus, by thermocompression bonding the thermal conductive sheet 1 to the adherend, the thermal conductive sheet 1 can be reliably fixed (temporary fixed) to the adherend.

Thus, by temporary fixing the thermal conductive sheet 1 in B-stage to the adherend, and thereafter, curing the thermal conductive sheet 1 by heating, the thermal conductive sheet 1 can be reliably adhered to the adherend, and the thermal conductive sheet 1 allows the heat from the adherend to be conducted efficiently along the plane direction SD of the thermal conductive sheet 1.

The adherend is not particularly limited, and examples thereof also include, in addition to the above-described electronic components (IC chips, condensers, coils, and resistors), light-emitting diodes.

On the other hand, sometimes there is a case where it is desired to remove once the thermal conductive sheet 1 for position adjustment as necessary after temporary fixing the thermal conductive sheet 1 to the adherend, and bonded thereto again (reworking) In this case, the above-described thermal conductive sheet 1 is in B-stage, and reworkability is excellent. Thus, residue of the thermal conductive sheet 1 on the surface of the adherend can be prevented at the time of removing, and at the same time, reworking of the thermal conductive sheet 1 is easy.

Furthermore, even if the thermal conductive sheet 1 remains on the surface of the adherend, as long as the thermal conductive sheet 1 is uncured (before curing) the residue can be easily wiped off (removed).

In the above-described hot-pressing step (FIG. 2( a)), for example, a plurality of calendering rolls and the like can be used for rolling the mixture and the laminated sheet 1C.

When the resin component 3 is the thermosetting resin component, without curing by heat as described above, the thermal conductive sheet can be obtained as the above-described uncured thermal conductive sheet 1.

That is, with the thermal conductive sheet of the present invention, when the resin component is the thermosetting resin component, there is no particular limitation as to whether or not curing by heat is carried out or when curing by heat is carried out. For example, the curing by heat can be performed after the laminating step (FIG. 2( c)) as described above, or can be performed after the elapse of a predetermined period from the above-described hot-pressing step (FIG. 2( a), hot-pressing of the mixture but the hot-pressing does not allow curing by heat). To be specific, the curing by heat can be performed at the time when the sheet is applied in power electronics technology, or after the elapse of a predetermined period after such application.

EXAMPLES

While the present invention will be described hereinafter in further detail with reference to Examples and Comparative Examples, the present invention is not limited to these Examples and Comparative Examples.

Example 1

Components (boron nitride particles and epoxy resin composition) were blended in accordance with the mixing formulation of Table 1, stirred, and allowed to stand at room temperature (23° C.) for one night, thereby allowing methyl ethyl ketone (solvent for curing agent/dispersion medium for curing agent) to volatilize and preparing a semi-solid mixture.

Then, the obtained mixture was sandwiched by two silicone-treated releasing films, and then these were hot-pressed with a vacuum hot-press at 80° C. under an atmosphere (vacuum atmosphere) of 10 Pa with a load of 5 tons (20 MPa) for 2 minutes. A pressed sheet having a thickness of 0.3 mm was thus obtained (ref: FIG. 2( a)).

Thereafter, the obtained pressed sheet was cut so as to be divided into a plurality of pieces when projected in the thickness direction of the pressed sheet. Divided sheets were thus obtained (ref: FIG. 2( b)). Next, the divided sheets were laminated in the thickness direction. A laminated sheet was thus obtained (ref: FIG. 2( c)).

Then, the obtained laminated sheet was hot-pressed under the same conditions as described above with the above-described vacuum hot-press (ref: FIG. 2( a)).

Then, a series of the above-described operations of cutting, laminating, and hot-pressing (ref: FIG. 2) was repeated four times. A thermal conductive sheet having a thickness of 0.3 mm was thus obtained.

Thereafter, the obtained thermal conductive sheet was introduced into a dryer, and heated at 150° C. for 120 minutes so as to be cured by heat.

Examples 2 to 8, 10 to 16, and Comparative Examples 1 and 2

Thermal conductive sheets having a thickness of 0.3 mm of Examples 2 to 8, 10 to 16, and Comparative Examples 1 and 2 were obtained in the same manner as in Example 1 in accordance with the mixing formulation and production conditions of Tables 1 to 3.

Example 9

A mixture was prepared by blending and stirring components (boron nitride particles and polyethylene) in accordance with the mixing formulation of Table 2. That is, during the stirring of the components, the mixture was heated to 130° C., and polyethylene was melted.

Then, the obtained mixture was sandwiched by two silicone-treated releasing films, and then these were hot-pressed with a vacuum hot-press at 120° C. under an atmosphere (vacuum atmosphere) of 10 Pa with a load of 1 ton (4 MPa) for 2 minutes. A pressed sheet having a thickness of 0.3 mm was thus obtained (ref: FIG. 2( a)).

Thereafter, the obtained pressed sheet was cut so as to be divided into a plurality of pieces when projected in the thickness direction of the pressed sheet. Divided sheets were thus obtained (ref: FIG. 2( b)). Next, the divided sheets were laminated in the thickness direction. A laminated sheet was thus obtained (ref: FIG. 2( c)).

Then, the obtained laminated sheet was hot-pressed under the same conditions as described above with the above-described vacuum hot-press (ref: FIG. 2( a)).

Then, a series of the above-described operations of cutting, laminating, and pressing (ref: FIG. 2) was repeated four times. A thermal conductive sheet having a thickness of 0.3 mm was thus obtained.

Evaluation (1) Thermal Conductivity

The thermal conductivity of the thermal conductive sheets obtained in Examples and Comparative Examples was measured.

That is, the thermal conductivity in the plane direction (SD) was measured by a pulse heating method using a xenon flash analyzer “LFA-447” (manufactured by Erich NETZSCH GmbH & Co. Holding KG). The thermal conductivity in the thickness direction (TD) was measured by a TWA method using “ai-Phase mobile” (manufactured by ai-Phase Co., Ltd).

The results are shown in Tables 1 to 3 (Examples 1 to 16, and Comparative Examples 1 and 2), and FIG. 8 (Examples 1 to 4, and Comparative Examples 1 and 2).

(2) Observation of Cross Section by Electron Microscope

The thermal conductive sheets of Examples 1, 3, and 5, and Comparative Examples 1 and 2 were cut along the thickness direction with a cross section polisher, and the cross sections thus made were observed with an electron microscope (SEM).

FIG. 3 to FIG. 7 show such processed images of SEM.

(3) Bend Resistance (Flexibility)

The bend test in conformity with JIS K 5600-5-1 bend resistance (cylindrical mandrel method) was carried out for the thermal conductive sheets of Examples and Comparative Examples.

That is, for the thermal conductive sheets of Examples 1 to 8, 10 to 16, and Comparative Examples 1 and 2, the laminated sheets having a thickness of 0.3 mm before curing were prepared as samples, and subjected to the bend test.

For the thermal conductive sheet of Example 9, the thermal conductive sheet thus obtained having a thickness of 0.3 mm was subjected to the bend test as is.

Thereafter, bend resistance (flexibility) of the thermal conductive sheets was evaluated under the test conditions shown below.

Test Conditions:

Test Device: Type I

Mandrel: diameter 10 mm

Then, the uncured thermal conductive sheets were bent to a bending angle of more than 0 degree and 180 degrees or less, and evaluated as follows based on the angle by which fracture (damage) is caused in the thermal conductive sheet.

The results are shown in Tables 1 to 3.

Excellent: No fracture was caused even if bent to 180 degrees.

Good: Fracture was caused when bent to 90 degrees or more and below 180 degrees.

Poor: Fracture was caused when bent to 10 degrees or more and below 90 degrees.

Bad: Fracture was caused when bent to more than 0 degree and below 10 degrees.

(4) Porosity (P)

The porosity (P1) of the thermal conductive sheet before curing in Examples and Comparative Examples was measured by the following method.

Measurement method of porosity: The thermal conductive sheet was cut along the thickness direction with a cross section polisher (CP); and the cross section thus appeared was observed with a scanning electron microscope (SEM) at a magnification of 200. The obtained image was binarized based on the pore portion and the non-pore portion; and the area ratio, i.e., the ratio of the pore portion area to the total area of the cross section of the thermal conductive sheet was calculated.

The results are shown in Tables 1 to 3.

(5) Conformability to Irregularities (3-Point Bending Test)

The 3-point bending test in conformity with JIS K 7171 (2010) was carried out for the thermal conductive sheets before curing by heat of Examples and Comparative Examples with the following test conditions, thus evaluating conformability to irregularities with the following evaluation criteria. The results are shown in Tables 1 to 3.

Test Conditions:

Test Piece: size 20 mm×15 mm

Distance Between Supporting Points: 5 mm

Testing Speed: 20 mm/min (indenter depressing speed)

Bending Angle: 120 degrees

(Evaluation Criteria)

Excellent: No fracture was observed.

Good: Almost no fracture was observed.

Bad: Fracture was clearly observed.

(6) Printed Mark Visibility (Mark Adhesion by Printing: Mark Adhesion by Inkjet Printing or Laser Printing)

Marks were printed on the thermal conductive sheet of Examples 1 to 16 by inkjet printing and laser printing, and the mark was observed.

As a result, it was confirmed that the mark was excellently visible in both cases of inkjet printing and laser printing, and that mark adhesion by printing was excellent in any of the thermal conductive sheets of Examples 1 to 16.

(7) Volume Resistivity

The volume resistivity (R) of the thermal conductive sheet in Examples and Comparative Examples was measured.

That is, the volume resistivity (R) of the thermal conductive sheet was measured in conformity with JIS K 6911 (testing methods for thermosetting plastics, 2006).

The results are shown in Tables 1 to 3.

(8) Electric Strength Test (JIS C 2110 (2010))

The breakdown voltage of the thermal conductive sheet obtained in Examples and in Comparative Examples was measured in conformity with JIS C 2110 (2010).

That is, the breakdown voltage was measured by a short period (rapid pressure rise) test with a pressure rising speed of 1000 V/s in conformity with the description of JIS C 2110-2 (2010) “Solid electrical insulating materials—Test methods for electric strength—Part 2: Tests using direct voltage”.

The results are shown in Tables 1 to 3.

(9) Glass Transition Temperature

The glass transition temperature of the thermal conductive sheet obtained in Examples and Comparative Examples was measured.

That is, the thermal conductive sheet was measured with a temperature rising speed of 1° C./min and a frequency of 10 Hz using a dynamic viscoelasticity measuring apparatus (model number: DMS 6100, manufactured by Seiko Instruments Inc.).

The glass transition temperature was determined by the obtained data, i.e., the peak value of tan δ.

The results are shown in Tables 1 to 3.

(10) Mass Loss Measurement

The 5% mass loss temperature of the thermal conductive sheet obtained in Examples and Comparative Examples was measured by thermogravimetric analysis (temperature-rising speed of 10° C./min, under nitrogen atmosphere) using a thermogravimetric analysis apparatus in conformity with JIS K 7120.

The results are shown in Tables 1 to 3.

(11) Initial Adhesion Test

A. Initial Adhesion Test for Mounting Substrate for Notebook PC

Initial adhesion tests (1) and (2) of the uncured thermal conductive sheet in Examples and Comparative Examples to a mounting substrate for notebook PC on which a plurality of electronic components are mounted were conducted.

That is, the thermal conductive sheet was temporary fixed to the surface (the side on which the electronic components are mounted) along the horizontal direction of the mounting substrate for notebook PC using a sponge roll made of silicone resin by thermocompression bonding at 80° C. (Examples 1 to 8 and Examples 10 to 16) or 120° C. (Example 9), and then allowed to stand for 10 minutes, and thereafter, the mounting substrate for notebook PC was disposed along the up-down directions (Initial Adhesion Test (2)).

Afterwards, the mounting substrate for notebook PC was positioned so that the thermal conductive sheet faces downward (that is, turned over to be upside down from the position of the temporary fixing) (Initial Adhesion Test (1)).

Then, in the above-described Initial Adhesion Test (1) and Initial Adhesion Test (2), the thermal conductive sheet was evaluated based on the criteria below. The results are shown in Tables 1 to 3.

<Criteria>

Good: It was confirmed that the thermal conductive sheet did not fall off from the mounting substrate for notebook PC.

Bad: It was confirmed that the thermal conductive sheet fell off from the mounting substrate for notebook PC.

B. Initial Adhesion Test to Stainless Steel Substrate

Initial adhesion tests (1) and (2) were conducted in the same manner as described above for adhesion of the uncured thermal conductive sheet of Examples and Comparative Examples to a stainless steel substrate (made of SUS 304).

Then, in the above-described Initial Adhesion Test (1) and Initial Adhesion Test (2), the thermal conductive sheet was evaluated based on the criteria below. The results are shown in Tables 1 to 3.

<Criteria>

Good: It was confirmed that the thermal conductive sheet did not fall off from the stainless steel substrate.

Bad: It was confirmed that the thermal conductive sheet fell off from the stainless steel substrate.

(12) Orientation Angle (α) of Boron Nitride Particles

The thermal conductive sheet was cut and processed along the thickness direction with a cross section polisher (CP), and the cross section thus made was photographed with a scanning electron microscope (SEM) at a magnification of 100 to 2000. The tilt angle (α) between the longitudinal direction (LD) of the boron nitride particles and the plane direction (SD) of the thermal conductive sheet was obtained based on the obtained processed images of SEM (ref: FIG. 3 to FIG. 7), and the orientation angle (α) of the boron nitride particles was calculated as its average value.

The results are shown in Tables 1 to 3.

(13) Kinetic Viscosity of Resin Component

The kinetic viscosity of the resin components used in Examples and Comparative Examples was measured by a kinetic viscosity test in conformity with JIS K 7233 (bubble viscometer method).

That is, first, the resin component and criterion products each was dissolved in a solvent (butyl carbitol) at a temperature of 25±0.5° C. so that the solid content concentration thereof is 40 mass %. A resin component sample and a criterion sample were thus prepared. The criterion samples were classified into A5 to A1, A to Z, and Z1 to Z10 based on their kinetic viscosities, and the kinetic viscosities corresponding to these classes are within the range of 0.005×10⁻⁴ m²/s to 1066×10⁻⁴ m²/s.

Then, the bubble rising speed of the resin component sample was compared with the bubble rising speed of the criterion sample (known kinetic viscosity), and the kinetic viscosity of the resin component sample was determined to be the kinetic viscosity of the criterion sample having a rising speed that matched with that of the resin component sample. The kinetic viscosity of the resin components was measured in this manner.

The results are shown in Tables 1 to 3.

TABLE 1 Average Particle Size Examples (μm) Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Mixing Boron Nitride PT-110*¹ 45 3.83 5.75 12.22 23 — 12.22 Formulation Particles/g*^(A)/ [40]   [50]   [68]   [80]   [68]   of [vol %]*^(B)/ [38.8] [48.8] [66.9] [79.2] [66.9] Components [vol %]*^(C) UHP-1*²  9 — — — — 12.22 — [68]   [66.9] Polymer Thermosetting Epoxy resin Epoxy Resin A*³ 3 3 3 3 3 — Resin Compositon (Semi-solid) Epoxy Resin B*⁴ — — — — — 1.5 (Liquid) Epoxy Resin C*⁵ — — — — — 1.5 (Solid) Epoxy Resin D*⁶ — — — — — — (Solid) Curing Agent*⁷ 3 3 3 3 3 3 (Solid Content in Grams) (0.15) (0.15) (0.15) (0.15) (0.15) (0.15) Curing Agent*⁸ — — — — — — (Solid Content in Grams) Thermoplastic Polyethylene*⁹ — — — — — — Resin Production Hot Temperature (° C.) 80 80 80 80 80 80 Conditions Pressing Number of Time(Times)*^(D) 5 5 5 5 5 5 Load(MPa)/(tons) 20/5 20/5 20/5 20/5 20/5 20/5 Evaluation Thermal Thermal Conductivity Plane Direction (SD) 4.5 6.0 30.0 32.5 17.0 30.0 Conductive (W/m · K) Thickness Direction (TD) 1.3 3.3 5.0 5.5 5.8 5.0 Sheet Ratio (SD/TD) 3.5 1.8 6.0 5.9 2.9 6.0 Flexibility/Bend Test JIS K 5600-5-1 Excel- Excel- Excel- Excel- Good Excel- lent lent lent lent lent Porosity (vol %) 0 0 5 12 6 4 Conformability to Irregularities/3-point Good Good Good Good Good Good Bending Test JIS K 7171 (2008) Volume Resistivity (Ω · cm) 5.5 × 3.4 × 2.1 × 1.3 × 1.7 × 2.2 × JIS K 6911(2006) 10¹⁴ 10¹⁴ 10¹⁴ 10¹⁴ 10¹⁴ 10¹⁴ Breakdown Voltage (kV/mm) 48 51 38 21 40 39 JIS C 2110 (2010) Glass Transition Temperature (° C.) 139 140 139 138 140 130 5% weight loss temperature JIS K 7120 318 327 333 381 341 344 Initial To Mounting Test (1) Good Good Good Good Good Good Adhesion Substrate for Test Notebook PC Test (2) Good Good Good Good Good Good To Stainless Test (1) Good Good Good Good Good Good Steel Substrate Test (2) Good Good Good Good Good Good Boron Nitride Orientation Angle (α)(degrees) 18 18 15 13 20 15 Particles Polymer Kinetic Viscosity of Corresponding A2~D A2~D A2~D A2~D A2~D A2~D Polymer Criterion Product JIS K 7233 Kinetic viscosity 0.22~ 0.22~ 0.22~ 0.22~ 0.22~ 0.22~ (Bubble Viscometer (×10⁻⁴ m²/s) 1.00 1.00 1.00 1.00 1.00 1.00 Method) g*^(A): Blended Weight [vol %]*^(B): Percentage relative to the Total Volume of the Thermal Conductive Sheet (excluding curing agent) [vol %]*^(C): Percentage relative to the Total Volume of the Thermal Conductive Sheet Number of Time*^(D): Number of times of hot-pressing of laminated sheet

TABLE 2 Average Particle Size Examples (μm) Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Mixing Boron Nitride PT-110*¹ 45 12.22 12.22 3.83 13.42 3.83 13.42 Formulation Particles/g*^(A)/ [68]   [68]   [60] [70] [40]   [70] of [vol %]*^(B)/ [66.9] [66.9] [60] [69] [37.7] [69] Components [vol %]*^(C) UHP-1*²  9 — — — — — — Polymer Thermosetting Epoxy resin Epoxy Resin A*³ — — — — 3 3 Resin Compositon (Semi-solid) Epoxy Resin B*⁴ 3 — — — — — (Liquid) Epoxy Resin C*⁵ — 3 — — — — (Solid) Epoxy Resin D*⁶ — — — 3 — — (Solid) Curing Agent*⁷ 3 3 — 3 6 3 (Solid Content in Grams) (0.15) (0.15) (0.15) (0.3) (0.15) Curing Agent*⁸ — — — — — — (Solid Content in Grams) Thermoplastic Polyethylene*⁹ — — 1 — — — Resin Production Hot Temperature (° C.) 80 80 120 80 80 60 Conditions Pressing Number of Time (Times)*^(D) 5 5 5 5 5 5 Load(MPa)/(tons) 20/5 20/5 4/1 20/5 20/5 20/5 Evaluation Thermal Thermal Conductivity Plane Direction (SD) 30.0 30.0 20 24.5 4.1 10.5 Conductive (W/m · K) Thickness Direction (TD) 5.0 5.0 2.0 2.1 1.1 2.2 Sheet Ratio (SD/TD) 6.0 6.0 10.0 11.7 3.7 4.8 Flexibility/Bend Test JIS K 5600-5-1 Good Poor Bad Good Excel- Good lent Porosity (vol %) 2 13 1 10 0 29 Conformability to Irregularities/3-point Good Bad Bad Bad Excel- Excel- Bending Test JIS K 7171 (2008) lent lent Volume Resistivity (Ω · cm) 2.4 × 1.1 × 4.1 × 1.3 × 6.4 × 0.6 × JIS K 6911(2006) 10¹⁴ 10¹⁴ 10¹⁴ 10¹⁴ 10¹⁴ 10¹⁴ Breakdown Voltage (kV/mm) 37 18 40 24 48 10 JIS C 2110 (2010) Glass Transition Temperature (° C.) 168 107 −24 217 145 138 5% weight loss temperature JIS K 7120 357 325 430 370 310 352 Initial To Mounting Test (1) Good Good Good Good Good Good Adhesion Substrate for Test Notebook PC Test (2) Good Good Good Good Good Good To Stainless Test (1) Good Good Good Good Good Good Steel Substrate Test (2) Good Good Good Good Good Good Boron Nitride Orientation Angle (α)(degrees) 16 16 15 16 20 17 Particles Polymer Kinetic Viscosity of Corresponding A5 Z — D~G A2~D A2~D Polymer Criterion Product JIS K 7233 Kinetic viscosity 0.005 5.00 — 1.00~ 0.22~ 0.22~ (Bubble Viscometer (×10⁻⁴ m²/s) 1.65 1.00 1.00 Method) g*^(A): Blended Weight [vol %]*^(B): Percentage relative to the Total Volume of the Thermal Conductive Sheet (excluding curing agent) [vol %]*^(C): Percentage relative to the Total Volume of the Thermal Conductive Sheet Number of Time*^(D): Number of times of hot-pressing of laminated sheet

TABLE 3 Examples and Comparative Examples Average Particle Size Comp. Comp. (μm) Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 1 Ex. 2 Mixing Boron Nitride PT-110*¹ 45 13.42 13.42 13.42 13.42 1.44 2.46 Formulation Particles/g*^(A)/ [70] [70] [70] [70] [20]   [30] of [vol %]*^(B)/ [69] [69] [69] [69] [19.3] [29] Components [vol %]*^(C) UHP-1*²  9 — — — — — — Polymer Thermosetting Epoxy resin Epoxy Resin A*³ 3 3 3 — 3 3 Resin Compositon (Semi-solid) Epoxy Resin B*⁴ — — — 1 — — (Liquid) Epoxy Resin C*⁵ — — — — — — (Solid) Epoxy Resin D*⁶ — — — 2 — — (Solid) Curing Agent*⁷ 3 3 3 — 3 3 (Solid Content in Grams) (0.15) (0.15) (0.15) (0.15) (0.15) Curing Agent*⁸ — — — 3 — — (Solid Content in Grams) (0.15) Thermoplastic Polyethylene*⁹ — — — — — — Resin Production Hot Temperature (° C.) 70 80 80 80 80 80 Conditions Pressing Number of Time(Times)*^(D) 5 5 5 5 5 5 Load(MPa)/(tons) 20/5 20/5 40/10 20/5 20/5 20/5 Evaluation Thermal Thermal Conductivity Plane Direction (SD) 11.2 32.5 50.7 30 1.5 1.1 Conductive (W/m · K) Thickness Direction (TD) 3.0 5.5 7.3 2.0 5.7 0.6 Sheet Ratio (SD/TD) 3.7 5.9 6.9 15.0 0.3 1.8 Flexibility/Bend Test JIS K 5600-5-1 Good Excel- Good Excel- Bad Bad lent lent Porosity (vol %) 26 8 3 4 0 0 Conformability to Irregularities/3-point Excel- Excel- Good Excel- Good Good Bending Test JIS K 7171 (2008) lent lent lent Volume Resistivity (Ω · cm) 0.8 × 2.5 × 5.3 × 2 × 8.2 × 7.9 × JIS K 6911(2006) 10¹⁴ 10¹⁴ 10¹⁴ 10¹⁴ 10¹⁴ 10¹⁴ Breakdown Voltage (kV/mm) 12 30 43 45 53 50 JIS C 2110 (2010) Glass Transition Temperature (° C.) 138 139 139 216 139 138 5% weight loss temperature JIS K 7120 348 355 350 362 280 285 Initial To Mounting Test (1) Good Good Good Good Good Good Adhesion Substrate for Test Notebook PC Test (2) Good Good Good Good Good Good To Stainless Test (1) Good Good Good Good Good Good Steel Substrate Test (2) Good Good Good Good Good Good Boron Nitride Orientation Angle (α)(degrees) 15 15 13 12 31 34 Particles Polymer Kinetic Viscosity of Corresponding A2~D A2~D A2~D A2~D A2~D A2~D Polymer Criterion Product JIS K 7233 Kinetic viscosity 0.22~ 0.22~ 0.22~ 0.22~ 0.22~ 0.22~ (Bubble Viscometer (×10⁻⁴ m²/s) 1.00 1.00 1.00 1.00 1.00 1.00 Method) g*^(A): Blended Weight [vol %]*^(B): Percentage relative to the Total Volume of the Thermal Conductive Sheet (excluding curing agent) [vol %]*^(C): Percentage relative to the Total Volume of the Thermal Conductive Sheet Number of Time*^(D): Number of times of hot-pressing of laminated sheet

In Tables 1 to 3, values for the components are in grams unless otherwise specified.

In the rows of “boron nitride particles” in Tables 1 to 3, values on the top represent the blended weight (g) of the boron nitride particles; values in the middle represent the volume percentage (vol %) of the boron nitride particles relative to the total volume of the solid content excluding the curing agent in the thermal conductive sheet (that is, solid content of the boron nitride particles, and epoxy resin or polyethylene); and values at the bottom represent the volume percentage (vol %) of the boron nitride particles relative to the total volume of the solid content in the thermal conductive sheet (that is, solid content of boron nitride particles, epoxy resin, and curing agent).

For the components with “*” added in Tables 1 to 3, details are given below.

-   PT-110*¹: trade name, plate-like boron nitride particles, average     particle size (light scattering method) 45 μm, manufactured by     Momentive Performance Materials Inc. -   UHP-1*²: trade name: SHOBN®UHP-1, plate-like boron nitride     particles, average particle size (light scattering method) 9 μm,     manufactured by Showa Denko K. K. -   Epoxy Resin A*³: OGSOL EG (trade name), bisarylfluorene epoxy resin,     semi-solid, epoxy equivalent 294 g/eqiv., softening temperature     (ring and ball test) 47° C., melt viscosity (80° C.) 1360 mPa·s,     manufactured by Osaka Gas Chemicals Co., Ltd. -   Epoxy Resin B*⁴: jER® 828 (trade name), bisphenol A epoxy resin,     liquid, epoxy equivalent 184 to 194 g/eqiv., softening temperature     (ring and ball test) below 25° C., melt viscosity (80° C.) 70 mPa·s,     manufactured by Japan Epoxy Resins Co., Ltd. -   Epoxy Resin C*⁵: jER® 1002 (trade name), bisphenol A epoxy resin,     solid, epoxy equivalent 600 to 700 g/eqiv., softening temperature     (ring and ball test) 78° C., melt viscosity (80° C.) 10000 mPa·s or     more (measurement limit or more), manufactured by Japan Epoxy Resins     Co., Ltd. -   Epoxy Resin D*⁶: EPPN-501HY (trade name), triphenylmethane epoxy     resin, solid, epoxy equivalent 163 to 175 g/eqiv., softening     temperature (ring and ball test) 57 to 63° C., manufactured by     NIPPON KAYAKU Co., Ltd. -   Curing Agent*⁷: a solution of 5 mass % Curezol® 2PZ (trade name,     manufactured by Shikoku Chemicals Corporation) in methyl ethyl     ketone. -   Curing Agent*⁸: a dispersion of 5 mass % Curezol® 2P4MHZ-PW (trade     name, manufactured by Shikoku Chemicals Corporation) in methyl ethyl     ketone. -   Polyethylene*⁹: low density polyethylene, weight average molecular     weight (Mw) 4000, number average molecular weight (Mn) 1700,     manufactured by Sigma-Aldrich Co.

While the illustrative embodiments of the present invention are provided in the above description, such is for illustrative purpose only and it is not to be construed as limiting the scope of the present invention. Modification and variation of the present invention that will be obvious to those skilled in the art is to be covered by the following claims. 

1. A thermal conductive sheet comprising a plate-like boron nitride particle, wherein the proportion of the boron nitride particle content is 35 vol % or more, and the thermal conductivity in a direction perpendicular to the thickness direction of the thermal conductive sheet is 4 W/m·K or more.
 2. The thermal conductive sheet according to claim 1, wherein the boron nitride particle has an average particle size as measured by a light scattering method of 20 μm or more.
 3. The thermal conductive sheet according to claim 1, wherein no fracture is observed in the thermal conductive sheet when the thermal conductive sheet is evaluated in a bend test in conformity with the cylindrical mandrel method of JIS K 5600-5-1 with the test conditions below: Test Conditions: Test Device: Type I Mandrel: diameter 10 mm Bending Angle: 90 degrees or more Thickness of the Thermal Conductive Sheet: 0.3 mm.
 4. The thermal conductive sheet according to claim 1, further comprising a resin component, wherein the resin component has a kinetic viscosity as measured by the kinetic viscosity test (temperature: 25° C.±0.5° C., solvent: butyl carbitol, solid content concentration: 40 mass %) in conformity with JIS K 7233 (bubble viscometer method) of 0.22×10⁻⁴ to 2.00×10⁻⁴ m²/s. 